The question at hand was whether Hall, who in 1978 helped assault and murder a 21-year-old woman, is intelligent enough to merit the death sentence. The court's decision could set new national standards for assessing the mental capacities of death row inmates. In 2002, the Supreme Court ruled that executing people who are intellectually disabled qualifies as cruel and unusual punishment, which is unconstitutional, but it left individual states to establish their own means of assessing a defendant's level of impairment.

Since the 2002 ruling, Florida has opted for a strict definition of intellectual disability as having a score of 70 or below on tests that measure a person’s IQ. The state says that Hall's average score puts him above a "bright line" of 70, and therefore makes him eligible to be executed. But Hall's lawyers and mental health organizations, including the American Psychological Association and American Psychiatric Association, argue that Hall's assessment does not include the standard 5-point margin of error built into the design of the test. If that uncertainty is considered, Hall would not be eligible for the death penalty, they argue.

In an hourlong hearing, justices grilled both Hall's and Florida's representatives over the statistical nuts and bolts of the IQ test and its analysis. Several expressed considerable confusion over what it means to have an intellectual disability (referred to by both lawyers and justices in the court transcript as "mental retardation"), given the phrase’s recent redefinition in the newest edition of the Diagnostic and Statistical Manual of Mental Disorders, or DSM. Justice Anthony Kennedy asked what it means to have an intellectual disability according to the DSM definition. Although such a definition means "that the scholars can talk about it," he asked: "Is … there any evidence that society in general gives substantial deference to the psychiatric profession in this respect?"

Other justices seemed to find the use of the IQ test alone problematic. "[T]he ultimate determination here is whether somebody is mentally retarded; and the IQ test is just a part of that," noted Justice Elena Kagan. "It's a part of one prong of that ultimate determination." In considering Florida's approach, she and other justices homed in on the fact that once a defendant received a test score above 70, the state does not allow them to submit other evidence about their cognitive abilities. "[W]hat your cutoff does is it essentially says the inquiry has to stop there," Kagan said. After a long line of cases demonstrating that "we allow people to make their best case about why they're not eligible for the death penalty," the Florida cutoff "stops that in its tracks," she said.

The court has until early summer to issue a decision.

One researcher, however, says Hall v. Florida demonstrates why the DSM’s definition of intellectual disability needed to be revised. Too often, says James Harris, a professor of neurodevelopmental psychiatry at Johns Hopkins University in Baltimore, Maryland, an IQ score can't predict or explain how a person copes with real-life challenges and social situations and is used inappropriately in court. That was one reason Harris spearheaded the recent changes in the DSM’s chapter on neurodevelopmental disorders and served as a consultant to Lee’s lawyers. The new DSM, Harris says, emphasizes "adaptive ability"—how a person responds to difficult social and practical challenges—and not just IQ.

The hearing went "pretty much as I expected," Lee Kovarsky of the University of Maryland Francis King Carey School of Law in Baltimore tells ScienceInsider. He represented Marvin Wilson, who had an IQ of 61, before he was executed for murder in Texas in 2012. Kovarsky says that although he and other advocates are hoping the court uses the case as an opportunity to issue a broad ruling on mental health and the death penalty, he says the justices appeared to take a narrower approach, focusing on the mechanics of the 70-and-below IQ rule. The court “wants to have its cake and eat it, too," he says. It "wants to say there's a bar" of mental function below which you cannot execute, but "it doesn't want to have to deal with the corollary, which is to define what mental retardation is."